A set-top terminal (STT) serves as a gateway between a user's television and a cable TV network delivering programming content. Such programming content may be delivered as a broadcast. It may also be delivered on an on-demand basis, for which services such as video on demand (VOD), subscription VOD, movies on demand, etc., are offered. In addition, a “network personal video recorder (NPVR)” service has been developed, allowing the user to perform trick mode functions (rewind, fast-forward, pause, etc.) on a presentation of programming content through use of a network. In fact, a network architecture and functionalities for implementing the NPVR service are described, e.g., in International Publication Number WO 2003/093944 published on Nov. 13, 2003. The NPVR service also allows a user to “reserve” past and future programs for his/her review, even if such reserved programs were not identified by the user before their broadcast.
An STT receives, through the cable TV network, programming content which may be encrypted, e.g., in accordance with the data encryption standard (DES) technique, to secure its delivery. DES is a well known symmetrical cipher which utilizes a single key for both encryption and decryption of messages. Because the DES algorithm is publicly known, learning the DES key would allow an encrypted message to be read by anyone. As such, both the message sender and receiver must keep the DES key a secret from others. A DES key typically is a sequence of eight bytes, each containing eight bits. To enhance the DES integrity, the DES algorithm may be applied successive times. With this approach, the DES algorithm enciphers and deciphers data, e.g., three times in sequence, using multiple keys, resulting in a so-called triple DES (3DES) technique.
In contrast to the DES technique, a public key encryption technique, e.g., an RSA technique (named for its developers, Rivest, Shamir, and Adleman), uses two different keys. A first key, referred to as a private key, is kept secret by a user. The other key, referred to as a public key, is available to anyone wishing to communicate with the user in a confidential manner. The two keys uniquely match each other, collectively referred to as a “public key-private key pair.” However, the private key cannot be easily derived from the public key. A party wishing to send a message to the user may utilize the public key to encrypt a message before transmitting it. The user then utilizes the private key to decrypt the message. Conversely, the private key may be used to encrypt a message, in which case the message can subsequently be decrypted with the public key. For example, the keys for the RSA algorithm are mathematically generated, in part, by combining prime numbers. The security of the RSA algorithm, and the like, depends on the use of very large numbers for its keys, which typically are 512 bits long.
Public key encryption methodologies may often be utilized for authentication purposes. For example, a first party wishing to authenticate a data file may apply a well-known hash function (such as the SHA-1 algorithm) to the file, producing a hash value, and encode the hash value using its private key, producing an encoded value. An encoded value generated in this manner is sometimes referred to as a digital signature. The first party transmits the data file, and the digital signature, to a second party. The second party may then utilize the first party's known public key to decode the digital signature, generating a decoded value. The second party additionally applies the known hash function to the data file received from the first party, generating a verification value. The decoded value is compared with the verification value; if the decoded value and the verification value match, the first party may be confident of the sender's identity.
The authentication technique described above is useful when the second party has knowledge of the first party's public key. However, in many instances, the second party may not have such knowledge, and therefore cannot perform the steps necessary to authenticate the first party's identity. A common solution to this problem is to use a registration message issued by a commonly-known, trusted entity. A registration message may be, for example, a digital certificate generated in accordance with the ISO/X.509 standards published by the International Organization for Standardization. A registration message has value if both the first and second parties trust the trusted entity and have knowledge of the trusted entity's public key. In such case, the first party may generate an “unsigned” message containing its public key, and provide the message to the trusted entity with a request that the trusted entity “sign” the message. The trusted entity applies a well-known hash function to all or a portion of the message, and uses its private key to encode the resulting hash value, generating a digital signature. The trusted entity appends the signature to the message, and returns the resulting registration message to the first party. The first party may subsequently provide the signed registration message to a second party, who utilizes the trusted entity's public key to verify the registration message and extract the first party's public key therefrom. A trusted entity which issues registration messages in the manner described above is sometimes referred to as a “trusted licensing authority.”
A licensing “hierarchy” may also be established with a trusted licensing authority as the highest (and trusted) authority. In a system using such a licensing hierarchy, a party may be required to maintain multiple registration messages establishing a chain of authority up to the licensing authority. To authenticate its identity, a party provides to the second party as many registration messages as is necessary to demonstrate that its identity is recognized within the licensing hierarchy.
In prior art, programming content may be encrypted using a DES key, in accordance with a DES algorithm, to secure its delivery from a headend of a cable TV system to an STT. In order for the STT to decrypt the encrypted programming content, the DES key is transmitted from the headend to the STT in an entitlement control message (ECM), which is encrypted using a 3DES key in accordance with a 3DES algorithm. The 3DES key (also known as a “multi-session key (MSK)”) is sent to the STT in a separate entitlement management message (EMM), which is encrypted using an STT public key in accordance with a public key algorithm, whose private key counterpart is securely maintained in the STT. Thus, after receiving the encrypted EMM and ECM, the STT decrypts the encrypted EMM using the STT private key to obtain the 3DES key therein. Using such a 3DES key, the STT decrypts the encrypted ECM to obtain the DES key therein. Using such a DES key, the STT can decrypt the encrypted programming content it received.
Recently, some STTs for cable TV were improved to incorporate digital video recorder (DVR) functions (“DVR STTs”). Like a DVR, e.g., a Tivo or ReplayTV device, a DVR STT typically includes a hard drive, e.g., a disk, for digitally recording TV programs. Also like a DVR, a DVR STT allows a cable TV subscriber to record his/her favorite TV programs for later review, and exercise a season-pass-like option to record every episode of his/her favorite program for a period. It may automatically record programs for the user based on his/her viewing habit and preferences. The presentation of the recorded programming content can be manipulated by exercising rewind, pause and fast-forward functions.
However, cable operators have observed that providing to subscribers unrestricted recording of content might result in an unacceptable amount of unauthorized copying and/or distribution. Accordingly, there is a continuing need for a strategy that allows content to be stored by a subscriber, but at the same time prevents (or controls) copying and distributing the content to unauthorized parties. A number of techniques have been developed to address this need. One such technique involves use of an indicator, e.g., an encryption mode indicator (EMI), which may be inserted into a data stream used to transmit content from a source device to a destination device. The EMI provides to the destination device information concerning the status of the content; the status may indicate that the content can be freely copied, copied once, never copied, etc. The destination device reads the EMI and determines whether or not the content may be copied. If copying is permitted, the destination device may then copy the content. For details on such a content protection technique, one may refer to: “5C Digital Transmission Content Protection White Paper,” Hitachi, Ltd et al., Revision 1.0, Jul. 14, 1998.
Another technique requires a device intending to transmit protected content to determine whether or not the receiving device is authorized to receive such content. One such technique is disclosed in “High-Bandwidth Digital Content Protection System,” Digital Content Protection LLC, Revision 1.1, Jun. 9, 2003. In accordance with the disclosed technique, both the transmitting device and the receiving device have a valid array of secret device keys and a corresponding key selection vector. During an authentication process, the two devices exchange key selection vectors. The receiving device uses the transmitting device's key selection vector to generate a selection of its own secret device keys, and then calculates a value Km by adding the selected secret device keys using 56-bit binary addition. The transmitting device calculates a corresponding value Km′ using the receiving device's key selection vector. If each device has a valid set of secret device keys, Km=Km′. Only after the receiving device has established its legitimacy does the transmitting device deliver the content.
Another strategy used to control the usage and distribution of protected content is to employ a digital rights management (DRM) system. An example of a DRM system is the Microsoft Windows Media digital rights management system (MS-DRM). According to this system, a digital media file is encrypted and locked with a “license key.” The license key is stored in a license file which is distributed separately from the media file. A customer may obtain the encrypted media file by, e.g., downloading it from a web site, purchasing it on a disk, etc. To play the digital media file, the customer must first acquire the license file containing the corresponding license key. The customer acquires the license key by accessing a pre-delivered license; alternatively, when the customer plays the file for the first time, a procedure is activated for retrieving the license via the Internet. After obtaining the license with the license key, the customer can play the media file according to the rules or rights specified in the license.
Another example of a DRM system is described in “RealSystem Media Commerce Suite (Technical White Paper),” which is incorporated herein by reference in its entirety. A content file is encrypted by the system operator to become a secured content file, requiring a key to play the content in the file. The key is imported into a retailer's database, and the secured content file is provided to consumers by, e.g., offline distribution of CDs. The retailer sets usage rules for licensing content. A customer obtains the secured content file and contacts the retailer's web server through a trusted client to obtain a license to play the content. The retailer's web server requests rights from the operator's license server, which creates a license containing the key for the respective content file, and provides the license to the retailer's web server. The retailer's web server delivers the license to the trusted client. The trusted client receives the license with the key, retrieves the content file, and uses the key to play the content.
In recent years, numerous systems for providing interconnectivity among devices in a home have been developed, allowing home networks to include cable STTs, personal computers, cellphones, PDA devices, etc. An example of a system for interconnecting various devices in a home is described in International Publication No. WO 02/21841, published on Mar. 14, 2003. Because of the increasing popularity of home networking, there is a growing need for a strategy that enables a user to perform authorized transfer of protected content, e.g., transferring content from an STT to a second device in a home network, and at the same time prevents unauthorized distribution of the protected content.
In addition, in the cable industry, a CableCARD (also known as a “a point-of-deployment (POD) module”) has been developed to satisfy certain security requirements to allow retail availability of host devices, e.g., set-top boxes, digital cable ready televisions, DVRs, personal computers (PCs), integrated digital televisions, etc., for receiving cable services. The CableCARD, comprising a PCMCIA device, can be inserted into a host device, allowing a viewer to receive cable systems' secure digital video services, e.g., pay per view TV, electronic program guides, premium subscription channels, etc.
Specifically, the CableCARD contains conditional access functionality, as well as the capability of converting messages to a common format. Thus, the CableCARD provides a cable operator with a secure device at the subscriber premises, and acts as a translator so that the host device needs to understand a single protocol, regardless of the type of the network to which it is connected.
For example, with the CableCARDs provided by cable operators, host devices which run, e.g., on an OpenCable Applications Platform (OCAP), may be sold in retail outlets. (For details on such a platform, one may refer, e.g., to: “OpenCable Application Platform Specification,” OCAP 2.0 Profile, OC-SP-OCAP2.0-I01-020419, Cable Television Laboratories, Inc., Apr. 19, 2002.) The OCAP allows applications to be built to a common middleware layer for deployment on host devices interoperable across cable systems in North America. (For details on the functional requirements of one such host device, one may refer, e.g., to: “OpenCable™ Host Device Core Functional Requirements,” OC-SP-HOSR-CFR-I13-030707, Cable Television Laboratories, Inc., Jul. 7, 2003.) With a common interface to the CableCARD, a host can be moved from one place to another, provided that the user of the host device contact his/her new cable operator to obtain a new CableCARD. (For details on such an interface, one may refer, e.g., to: “OpenCable™ HOST-POD Interface Specification,” OC-SP-HOSTPOD-IF-I13-030707, Cable Television Laboratories, Inc. Jul. 7, 2003. To provision a new CableCARD and host device, an initialization and authorization process needs to be performed while the host device, with the CableCARD inserted therein, is connected to the cable network. The initialization and authorization process begins with the user's providing an ID(s) of the CableCARD and/or the host device (e.g., serial number(s)) to the cable operator. The cable operator looks up in a database a MAC address of the CableCARD which typically is hard-coded in the CableCARD, and is associated with the CableCARD ID. During the authorization process, the cable operator may, for example, assign an IP address to the CableCARD for its identification in the cable network. The cable operator may also collect from the host device data concerning the make, model, and ID of the host device (e.g., its serial number). The cable operator may associate the CableCARD's MAC address (and/or IP address) with the user information, e.g., his/her name, address, etc. for billing purposes.